Ovulation Calculator
Estimate ovulation day, fertile window, and next period from cycle inputs.
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Result
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Formula
The model projects next period then counts backward by luteal length to estimate ovulation.
Worked Example
Understanding Your Fertile Window
- Ovulation is estimated by subtracting your luteal phase length (typically 14 days) from your cycle length
- The fertile window includes about 5 days before ovulation (sperm can survive up to 5 days) plus the day of ovulation
- Cycle regularity significantly affects prediction accuracy. Very irregular cycles make calendar-based prediction unreliable
- Ovulation predictor kits (OPKs) detect the LH surge and provide more accurate real-time ovulation confirmation
This calculator provides estimates based on average cycle patterns. For the most accurate ovulation detection, combine calendar methods with OPKs or basal body temperature tracking.
You can also calculate changes using our Pregnancy Due Date Calculator, IVF Calculator or Gestational Age Calculator.
Frequently Asked Questions
How accurate is ovulation prediction?
Accuracy depends on cycle regularity. For consistent 28-day cycles, the estimate is fairly reliable. For irregular cycles, actual ovulation may differ by several days from the prediction.
Does this replace ovulation testing?
No. This is a calendar-based estimate. Ovulation predictor kits detect the actual LH surge and provide more reliable real-time confirmation of ovulation timing.
What is the fertile window?
The fertile window is the period when conception is possible: approximately 5 days before ovulation (sperm viability) through 12 to 24 hours after ovulation (egg viability).
What is the luteal phase?
The luteal phase is the time between ovulation and the start of your next period. It is typically 12 to 16 days and remains relatively consistent for each individual, which is why it is used to estimate ovulation.
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